During the past several years, the popularity and viability of fuel cells for producing both large and small amounts of electricity has increased significantly. Fuel cells conduct an electrochemical reaction with reactants such as hydrogen and oxygen to produce electricity and heat. Fuel cells are similar to batteries, except that fuel cells can be “recharged” while providing power. In addition, fuel cells are cleaner than other sources of power, such as devices that combust hydrocarbons.
Fuel cells provide a DC (direct current) voltage that may be used to power almost any electrical device, including motors, lights, computers, etc. A typical fuel cell includes an electrolyte disposed between two electrodes: an anode and a cathode. There are several different types of fuel cells, each using a different chemistry. Fuel cells are usually classified by the type of electrolyte used. For example, fuel cells are generally categorized into one of five groups: proton exchange membrane (PEM) fuel cells, alkaline fuel cells (AFC), phosphoric-acid fuel cells (PAFC), solid oxide fuel cells (SOFC), and molten carbonate fuel cells (MCFC).
While all fuel cells have some desirable features, solid oxide fuel cells (SOFC) have a number of distinct advantages over other fuel cell types. Some advantages of SOFCs include reduced problems with electrolyte management, increased efficiencies over other fuel cell types (SOFCs are up to 60% efficient), higher tolerance to fuel impurities, and the possible use of internal reforming or direct utilization of hydrocarbon fuels to produce, for example, hydrogen and methane.
Most SOFCs include an electrolyte made of a solid-state material such as a fast oxygen ion conducting ceramic. In order to provide adequate ionic conductivity in the electrolyte, SOFCs typically operate in the 500 to 1000 C temperature range. As noted above, the electrolyte is disposed between two electrodes: an anode and a cathode. An oxidant such as air is fed to the cathode that supplies oxygen ions to the electrolyte. A fuel such as hydrogen or methane is fed to the anode where the fuel reacts with oxygen ions transported through the electrolyte. This reaction produces electrons, which are then delivered to an external circuit as useful power.
Throughout the operation of an SOFC, the fuel cell is often cycled between room temperature and its full operating temperature. This thermal cycling causes the housing materials to contract and expand according to their coefficients of thermal expansion. This expansion and contraction introduces thermal stresses that may be transferred through the seals and other structural components directly to the ceramic cell. These thermal stresses effectively reduce the service life of an SOFC by compromising the seals or breaking the structurally brittle ceramic cells. Some designs have attempted to minimize the thermal stresses introduced by thermal cycling by the use of compliant members such as springs. However, such springs have a tendency to stress relieve at elevated temperature. To avoid this stress relief, super alloys are often used. These super alloys are expensive and thereby limit the applicability of fuel cell systems that make use of them.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.